1. Field of the Invention
The present invention relates to a sheet supply device for use in an image forming apparatus such as a copying machine, a printer, a facsimile apparatus or the like.
2. Related Background Art
For use in an image forming apparatus such as a copying machine, a printer, a facsimile apparatus or the like, there is known a sheet supplying cassette provided therein with reference guide members for one-side sheet supplying, at the upstream and downstream positions on a lateral face of the main body of the cassette, along the sheet supplying direction, for impinging on a lateral side, along the sheet supplying direction, of the sheets stacked in said main body.
FIG. 19 illustrates a conventional sheet supplying cassette, wherein, in a cassette main body 1, there is provided an inner plate 2 for supporting stacked sheets S. Said inner plate 2 is rendered vertically rotatable with respect to the main body 1, about end shafts 3 and is biased upwards by a coil spring 5. Said spring 5 is positioned at a side of the front end of the inner plate 2, corresponding to a sheet supplying roller provided in the main body of the image forming apparatus.
Also in the cassette main body 1, at the upstream and downstream sides of a lateral face along the sheet supplying direction, there are formed reference guide members 6, 7 for one-side sheet supplying, for impinging on a lateral side, along the sheet supplying direction, of the sheets S stacked in said cassette main body 1. Conventionally, the reference guide member 7 at the downstream side is provided at the same reference position as that of the reference guide member 6 at the upstream side, as shown in FIG. 20.
Also in the cassette main body 1, at the other lateral side, along the sheet supplying direction, there is provided a side defining spring (plate spring) 9 for pressing a lateral side, along the sheet supplying direction, of the stacked sheets S toward the cassette reference guide members 6, 7. On both sides of a rear lateral plate 10 of the cassette main body 1, there are provided rear impinging portions 10A, 10B for impinging on the rear end of the sheets S stacked in the cassette main body 1. Also in the vicinity of the front end of the inner plate 2 in the cassette main body 1, there is provided a separating finger 11, only at the side of the coil spring 5, for individually separating the sheets S stacked in the cassette main body 1.
The sheets S are loaded in stacked state in the following manner, into the cassette main body 1 of the above-explained structure.
At first the rear end of a sheet stack is made to impinge on the rear impinging portions 10A, 10B. Then the front end of said sheet stack is inserted between the inner plate 2 and the separating finger 11, with the depression of said inner plate 2. Then a lateral side, along the sheet supplying direction, of the sheet stack placed on the inner plate 2 is pressed toward the cassette guide members 6, 7 (FIG. 21 shows a state of the sheet stack loaded in the cassette main body 1 in this manner). The lateral side 12, along the sheet supplying direction, of the sheets S stacked in the cassette main body 1 is pressed toward the cassette reference guide members 6, 7 by the biasing force of the side defining spring 9.
Said stacked sheets S are advanced by a sheet supplying roller 13 of the main body of the image forming apparatus, rotating in contact with the top of said stack, and are separated one by one by said separating finger 11 and fed to the image forming unit.
Such use of a sheet supplying roller and a separating finger deviated to a side aims at reducing the number of component parts and providing a less expensive apparatus.
However, such conventional sheet supplying cassette may result in a skewed supply of the sheet S in the course of supply by the roller 13, as will be explained in the following in relation to FIGS. 22A to 22D.
When the sheet S is given a transporting force by the roller 13, a portion a of the sheet S precedes and a portion b is delayed by a resistance in transportation (FIG. 22A). With the progress of transportation of the sheet S, there is generated a bend around a portion d of the sheet S (FIG. 22B). With further progress of the sheet transportation, there is generated a situation where the sheet S is in contact with the cassette reference guide member 6 of the upstream side but not with the guide member 7 of the downstream side (FIG. 22C). In this state the bend around the portion d disappears, but the front end of the sheet S becomes inclined by t, with respect to the reference position. If the sheet S is advanced in this state, it is supplied in a skewed state from the cassette main body 1.
As a result, the toner image is transferred in an inclined position onto the sheet S, so that the precision of printing is deteriorated.